Mechanistic studies of peptide oxazolone racemization

Abstract

The racemization and ring-opening reactions of the oxazolone from benzyloxycarbonylaminoisobutyryl- l-phenylalanine with a number of amino acid esters were studied in several commonly used peptide solvents. These studies show that oxazolone racemization is a pseudo first-order reaction when the rate of racemization is much larger than the rate of ring-opening. The ring-opening reaction always proceeds via second-order kinetics. When the rates of racemization and ring-opening are comparable, oxazolone racemization follows second-order kinetics. Since oxazolone racemization probably involves removal of a proton to produce two charged ions, anionic chloride or phosphate ions accelerate the rate of racemization apparently by increasing the ionic strength of the solution. Solvents such as dioxane which can accommodate charge separation by solvation gives rise to much higher extents of oxazolone racemization than solvents such as toluene in which charge separation is not well tolerated. Perhaps the most important factor controlling the racemization process is the nucleophilicity-basicity ratio of the attacking nucleophile. Of the amino acid esters studied, ethyl glycinate has the most desirable nucleophilicity-basicity ratio. In toluene solution the coupling of this amino acid ester with the oxazolone from benzyloxycarbonylaminoisobutyryl- l-phenylalanine gives the unexpectedly high yield of approximately 75% optically active tripeptide. Under similar conditions methyl aminoisobutyrate, a very hindered nucleophile, gives completely racemized tripeptide. When a large excess of hydrazine hydrate is allowed to react with the oxazolone from benzyloxycarbonylaminoisobutyryl- l-phenylalanine an optically pure hydrazide is obtained which is identical to that obtained in the traditional manner from the optically active dipeptide ester and hydrazine hydrate. Hydrazine hydrate therefore has an extremely high nucleophilicity-basicity ratio which we attribute to the so-called “alpha effect”.